1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device and more particularly, to a method for fabricating a semiconductor device using a boron/phosphorus silicate glass (hereinafter referred to simply as BPSG) as an insulating layer between conductive layers or between a conductive layer and a gate electrode the formation of a smooth BPSG layer on a semiconductor substrate.
2. Description of the Prior Art
A BPSG film is smoothed by thermal treatment at relatively low temperatures and has good moisture proofing. Accordingly, the BPSG film has been often used in recent years as an insulating film of semiconductor devices and has been reported, for example, in "Solid State Technology, the Japanese edition, December 1986, pp. 54-59", "J. Electrochem. Soc. Solid-state Science and Technology, pp. 1517-1518", and Japanese Laid-open Patent Application No. 53-107271.
However, when a 7000 angstrom thick BPSG film having a phosphorus concentration of 7 wt % and a boron concentration of 3.1 wt % is thermally treated, as is particularly shown in FIG. 1, grains grow on the surface of the BPSG film, each having approximately a diameter of 3 micrometers and a height of 1.5 micrometers.
FIG. 1 shows the condition that the BPSG film is thermally treated at 800.degree. C. for 5 minutes in an atmosphere of N.sub.2 and then heated at a rate of 5.degree. C./minute for 30 minutes in an atmosphere of O.sub.2. When the temperature reaches 950.degree. C., the film is maintained at this temperature for 15 minutes in an atmosphere of N.sub.2 and then the temperature is decreased at a rate of 2.5.degree. C./minute for 60 minutes in an atmosphere of N.sub.2. When the temperature reaches 800.degree. C., the film is thermally treated at this temperature for 5 minutes in an atmosphere of N.sub.2. It will be noted that the flow rates of the N.sub.2 and O.sub.2 gases in the above thermal treatment are, respectively, 9 liters/minute.
The grains have undesirably the possibility of causing breakage of a conductive layer formed on the BPSG film. Moreover, the grains may impede formation of contact holes in the BPSG film because of the difference in etching rate between the grains and the BPSG film.